The recent increase in the use of portable electronic devices such as mobile telephones and notebook computers and the emerging trend of using rechargeable batteries in hybrid electric vehicles has created a need for smaller, lighter, longer lasting rechargeable batteries to provide the power to the above mentioned and other battery powered devices. During the 1990s, lithium rechargeable batteries, specifically lithium-ion batteries, became popular and, in terms of units sold, now dominate the portable electronics marketplace and are set to be applied to new, cost sensitive applications. However, as more and more power hungry functions are added to the above mentioned devices (e.g. cameras on mobile phones), improved and lower cost batteries that store more energy per unit mass and per unit volume are required.
The basic composition of a conventional lithium-ion rechargeable battery cell including a graphite-based anode electrode is shown in FIG. 1. The battery cell includes a single cell but may also include more than one cell.
The battery cell generally comprises a copper current collector 10 for the anode and an aluminium current collector 12 for the cathode, which are externally connectable to a load or to a recharging source as appropriate. A graphite-based composite anode layer 14 overlays the current collector 10 and a lithium containing metal oxide-based composite cathode layer 16 overlays the current collector 12. A porous plastic spacer or separator 20 is provided between the graphite-based composite anode layer 14 and the lithium containing metal oxide-based composite cathode layer 16 and a liquid electrolyte material is dispersed within the porous plastic spacer or separator 20, the composite anode layer 14 and the composite cathode layer 16. In some cases, the porous plastic spacer or separator 20 may be replaced by a polymer electrolyte material and in such cases the polymer electrolyte material is present within both the composite anode layer 14 and the composite cathode layer 16.
When the battery cell is fully charged, lithium has been transported from the lithium containing metal oxide via the electrolyte into the graphite-based layer where it has reacted with the graphite to create the compound, LiC6. The maximum capacity of such an anode is 372 mAh per gram of graphite. It will be noted that the terms “anode” and “cathode” are used in the sense that the battery is placed across a load.
It is well known that silicon can be used instead of graphite as the active anode material (see, for example, Insertion Electrode Materials for Rechargeable Lithium Batteries, M. Winter, J. O. Besenhard, M. E. Spahr, and P. Novak in Adv. Mater. 1998, 10, No. 10). It is generally believed that silicon, when used as an active anode material in a lithium-ion rechargeable cell, can provide a significantly higher capacity than the currently used graphite. Silicon, when converted to the compound Li21Si5 by reaction with lithium in an electrochemical cell, has a maximum capacity of 4,200 mAh/g, considerably higher than the maximum capacity for graphite. Thus, if graphite can be replaced by silicon in a lithium rechargeable battery, a substantial increase in stored energy per unit mass and per unit volume can be achieved.
In order that the anode material will reversibly react with lithium to charge and discharge the cell, the silicon in the anode should be composed of small particles, which can have any suitable shape, e.g. particles, fibres, structoids or pillared particles (particles that have columns or pillars formed on their surface usually by etching). From WO2007/083155, it is known that the particles will preferably have: (a) a high aspect ratio, i.e. the ratio of the largest dimension to the smallest dimension of a particle, which is preferably about 100:1, (b) a minor dimension (the smallest dimension of the particle) of around 0.08-0.5 μm and (c) a major dimension (the largest dimension of the particle) of the order of 20-300 μm. The high aspect ratio helps in accommodating the large volume change during lithiation and delithiation of the anode, i.e. during charging and discharging of the battery without physically breaking up the particle; this volume change may be of the order of 300%. However, the formation of such small crystalline forms is time-consuming and expensive.
One known method of making silicon anode material is by selective etching of silicon-based materials to create silicon pillars. One such approach is described in U.S. Pat. No. 7,033,936, which is incorporated herein by reference. According to this document, pillars are fabricated by creating a mask by depositing hemispherical islands of caesium chloride on a silicon substrate surface, covering the substrate surface, including the islands, with a film, and removing the hemispherical structures (including the film covering them) from the surface to form a mask with exposed areas where the hemispheres had been. The substrate is then etched in the exposed areas using reactive ion etching and the resist is removed, e.g. by physical sputtering, to leave an array of silicon pillars in the unetched regions, i.e. in the regions between the locations of the hemispheres.
An alternative, chemical approach is described in WO2007/083152 in the name of the present applicants. According to this method, silicon is etched using a solution containing HF and AgNO3. The mechanism postulated is that isolated nanoclusters of silver are electrolessly deposited on the silicon surface in an initial stage. In a second stage, the silver nanoclusters and the areas of silicon surrounding them act as local electrodes that cause the electrolytic oxidation of the silicon in the areas surrounding the silver nanoclusters to form SiF6 cations, which diffuse away from the etching site to leave the silicon underlying the silver nanocluster in the form pillars. The etched silicon can either be removed from the silicon substrate or used while still attached to the substrate.
The etching of silicon to form anode material is expensive to carry out and involves highly corrosive materials such as HF, which are difficult to handle.
Other methods for making the same type of high aspect ratio structures have also been presented in the literature including electrochemical etching of silicon, and deposition of fine Si structures using CVD, PECVD and sputtering, or SLS or VLS deposition such as VLS using Au catalysts. All these methods require varying levels of expense but all are above that of the method of the present invention.
Aluminium-silicon alloys are hard and wear-resistant with excellent cast-ability, weld-ability and low shrinkage and are used in very large quantities industrially wherever these properties are required, e.g. in car engine blocks and cylinder heads.